Doxorubicin is an anthracycline antibiotic isolated from a soil microorganism. Its anti-tumor effects are related to interactions with the enzyme topoisomerase-2 and production of double strand DNA breaks. In addition, this agent generates intracellular free radicals that are highly cytotoxic. Doxorubicin is considered one of the most broadly active antitumor agents. Not only is Doxorubicin typically considered an important element in modem therapy of breast, soft tissue sarcomas and other solid tumors, it is thought to be an important element of curative combination chemotherapy for acute leukemia, Hodgkin's disease, non-Hodgkin's lymphoma, and many childhood cancers. Thus, for many individuals with advanced stages of cancer, Doxorubicin serves as an important part of their medical regimen.
Administration of Doxorubicin therapy is generally limited in adults and children by a cumulative dose dependent cardiotoxicity. Irreversible cardiomyopathy with serious congestive heart failure can be a significant risk in patients who receive doses in excess of 500-550 mg/m2. Unfortunately, the dose that precipitates congestive heart failure varies widely (ranging from 30-880 mg/m2in a report of 1487 patients studied over a seven year period). Those subjects with advanced age or mild reductions in left ventricular systolic function at rest (left ventricular ejection fraction [LVEF]≦50%), are at greatest risk. In western industrialized countries, it is typically older subjects with cancer and some degree of underlying heart disease whom often are in greatest need for Doxorubicin therapy, but for whom medication may be withheld due to potential cardiotoxicity.
One method for detection of Doxorubicin-induced cardiomyopathy is intramyocardial biopsy with concomitant left and right ventricular pressure measurements made during cardiac catheterization. Unfortunately, this method involves an invasive procedure and may not be well suited for repetitive measurements over time. Radionuclide ventriculography is also widely used to screen those individuals at risk for developing Doxorubicin-induced cardiomyopathy. Individuals who develop a reduction in LVEF of 10% or greater or those individuals who have a fall in ejection fraction to lower than 50% during treatment are at greatest risk for developing irreversible cardiotoxicity. While this information is useful as a potential screening technique, for some individuals, the drop observed in LVEF occurs too late to avert the development of irreversible cardiomyopathy. For this reason, the total dose of Doxorubicin may be unduly limited for patients receiving chemotherapy. Importantly for many individuals, Doxorubicin therapy is often stopped before patients derive maximal benefit of the drug regimen. A noninvasive, widely available method for accurately detecting those individuals whom go on to develop cardiotoxicity would have marked clinical utility.
During the past 7 years, investigators have established the utility of MRI for identifying necrotic tissue within the left ventricle in patients sustaining myocellular injury. This technique incorporates the acquisition of gradient-echo pulse sequences with nonselective preparatory radiofrequency pulses after intravenous administration of Gadolinium chelates. In regions of myocardial necrosis, heightened signal intensity occurs on images collected 20 minutes after contrast administration that corresponds to expansion of extracellular volume due to myocellular membrane disruption and increased capillary permeability. This methodology has been utilized to identify transmural myocellular necrosis in patients sustaining acute or chronic Q-wave (ST-segment elevation), and subendocardial (non-transmural) injury in patients sustaining a non-Q-wave (non ST-segment elevation) myocardial infarction. The amount of necrosis found during MRI displays an inverse relationship with recovery of systolic thickening after coronary arterial revascularization. The absence of Gadolinium hyperenhancement 20 minutes after contrast administration is associated with myocardial viability and subsequent improvement in left ventricular contraction after sustaining a ST-segment or non ST-segment elevation myocardial infarction. Although some felt delayed enhancement techniques may overestimate regions of myocellular necrosis in the acute infarct, recently, a tagging study in animals indicated that delayed enhancement techniques do identify early myocellular necrosis after myocardial infarction (MI). It is believed that, in border zones of infarcts, dead cells may move due to tethering from adjacent live regions.
With MRI, cardiac structure can be imaged and LV function directly assessed with high temporal and spatial resolution. Since acoustic windows do not limit image acquisition, the utility of MRI is high particularly in subjects with a large or unusual body habitus. This heightened clarity of the images allows investigators to perform quantitative measures of LV structure and function with higher precision than that achieved with radionuclide and ultrasound techniques. A 5% change in LVEF in patients with reduced LV function can be detected with 90% power at a p-value of 0.05 with a sample size of 5 patients per group in a parallel study design. Depending upon operator experience, the same 5% change in LVEF requires an echocardiographic assessment of>100 subjects per group in the same study design. Similarly, the heightened spatial resolution (1 mm2 pixel sizes) achieved with delayed enhancement MRI techniques allows for the detection of micro-infarcts that heretofore may have only been appreciated as cardiac enzymatic elevations detected in serum samples, but not visualized with radionuclide or echocardiographic techniques.
In delayed enhancement imaging a contrast agent is administered to a patient and an image is acquired after the contrast agent has had an opportunity to be distributed to area that is to be imaged such that the contrast agent remains in injured tissue but does not remain in healthy tissue. Such delayed enhancement imaging may be used, for example, to identify myocardial infarcts as the necrotic tissue of the infarct region will retain the contrast agent while the contrast agent will be purged from the healthy tissue. As such, the infarct may appear as a localized region of higher intensity. Conventionally, delayed enhancement imaging may be used to identify localized regions of tissue damage in tissues such as cardiac tissue, brain tissue, nerve tissue or the like.